BUG: timedelta64 + Timestamp raises

```
>>> np.timedelta64(3600*10**9, 'ns') + pd.Timestamp.now()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: ufunc add cannot use operands with types dtype('<m8[ns]') and dtype('O')
```

I think we can fix this by defining `Timestamp.__array_priority__`